This invention relates to a fluid operated control system for a variable displacement pump or pumps driven by a prime mover such as an internal combustion engine. More particularly, the invention is directed to a system for controlling the per cycle displacement of a pump or pumps supplying fluid under pressure to implement actuators, as in a materials handling vehicle such as an excavator, in accordance with the load being imposed on the pump or pumps.
The use of variable displacement pumps is common in earth movers or like vehicles for powering various implement actuators on such vehicles. A constant torque control of the variable displacement pumps makes possible the most effective use of the output horsepower of the engine driving the pumps. Typical of known methods for the constant torque control of the pumps is the one wherein the pump displacement is controlled by the resultant of the delivery pressure of the pumps and the force of a spring or springs opposing the pump pressure.
The prior art devices constructed in accordance with the above constant torque control method have had in common the weakness that the spring means for providing the force required for controlling pump displacement are too bulky for the provision of compact pump control devices desired today. Such bulky spring means give rise to additional drawbacks such as great hysteresis and fluctuations in control performance. Pumps have been growing smaller and smaller in recent years by virtue of advancements in materials and design engineering. However, as the pump control devices have not been reduced in size because of the bulky spring means, neither have been the pump assemblies incorporating such devices.